1. Field of Art
The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits. Still more particularly, the disclosure relates to percussion hammer drill bits including a driver sub that is rotationally locked relative to a casing.
2. Background of Related Art
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drill string simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
Referring to FIGS. 1-3, a conventional percussion drilling assembly 10 for drilling through formations of rock to form a borehole is shown. Assembly 10 is connected to the lower end of a drillstring 11 (FIG. 3) and includes a top sub 20, a driver sub 40, a tubular case 30 axially disposed between top sub 20 and driver sub 40, a piston 35 slidably disposed in the tubular case 30, and a hammer bit 60 slidingly received by driver sub 40. A feed tube 50 extends between top sub 20 and piston 35.
The upper end of top sub 20 is threadingly coupled to the lower end of drillstring 11 (FIG. 3), and the lower end up top sub 20 is threadingly coupled to the upper end of case 30. Further, the lower end of case 30 is threadingly coupled to the upper end of driver sub 40. Hammer bit 60 slideably engages driver sub 40. A series of generally axial mating splines 61, 41 on bit 60 and driver sub 40, respectively, allow bit 60 to move axially relative to driver sub 40 while simultaneously allowing driver sub 40 to rotate bit 60 with drillstring 11 and case 30.
Hammer bit 60 is generally cylindrical in shape and includes a radially outer skirt surface 62 aligned with or slightly recessed from the borehole sidewall and a bottomhole facing cutting or bit face 64. The earth disintegrating action of the hammer bit 60 is enhanced by providing a plurality of cutting elements (not shown) that extend from the cutting face 64 for engaging and breaking up the formation. The cutting elements are typically inserts formed of a superhard or ultrahard material, such as polycrystalline diamond (PCD) coated tungsten carbide and sintered tungsten carbide, that are press fit into undersized apertures in bit face.
A guide sleeve 32 and a bit retainer ring 34 are also positioned in case 30 axially above driver sub 40. Guide sleeve 32 slidingly receives the lower end of piston 35. Bit retainer ring 34 is disposed about the upper end of hammer bit 60 and prevents hammer bit 60 from falling out of and completely disengaging driver sub 40.
A retainer sleeve 70 is coupled to driver sub 40 and extends along the outer periphery of hammer bit 60. Retainer sleeve 70 generally provides a secondary catch mechanism that allows the lower enlarged head of hammer bit 60 to be extracted from the wellbore upon lifting of the drill string 11 and percussion drilling assembly 10 in the event of a crack or break in the shank (rotational drive) section of bit 60.
During drilling operations, a compressed fluid (e.g., compressed air, compressed nitrogen, etc.) is delivered down the drill string 11 from the surface to percussion drilling assembly 10. In most cases, the compressed fluid is provided by one or more compressors at the surface. The compressed fluid serves to actuate piston 35 within case 30. As piston 35 moves reciprocally within case 30, it cyclically impacts hammer bit 60, which in turn cyclically impacts the formation to gouge, crush, and break the formation with the cutting elements mounted thereon. The compressed fluid ultimately exits the bit face 64 and serves to flush cuttings away from the bit face 64 to the surface through the annulus between the drill string and the borehole sidewall.
In addition, during drilling operations, drill string 11 and drilling assembly 10 are rotated. Mating splines 41, 61 on driver sub 40 and bit 60, respectively, allow bit 60 to move axially relative to driver sub 40 while simultaneously allowing driver sub 40 to rotate bit 60 with drillstring 11. As a result, the drill string rotation is transferred to the hammer bit 60. Rotary motion of the drill string 11 may be powered by a rotary table typically mounted on the rig platform or top drive head mounted on the derrick. The rotation of hammer bit 60 allows the cutting elements of bit 60 to be “indexed” to fresh rock formations during each impact of bit 60, thereby improving the efficiency of the drilling operation. Without indexing, the cutting structure extending from the lower face 64 of the hammer bit 60 may have a tendency to undesirably impact the same portion of the earth as the previous impact. Experience has demonstrated that for an eight inch hammer bit (e.g., hammer bit 60), a rotational speed of approximately 20 rpm and an impact frequency of 1600 bpm (beats per minute) typically result in relatively efficient drilling operations. This rotational speed translates to an angular displacement of approximately 5 to 10 degrees per impact of the bit against the rock formation.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense.
As previously described, in most conventional bits, the driver sub 40 is threadingly coupled to the lower end of the case 30. During drilling, repeated impacts and vibration of the percussion drilling assembly 10 occasionally results in the inadvertent unthreading of the driver sub 40 from the case 30, resulting in the complete disengagement of the driver sub 40 and the drill bit 60 from the remainder of the percussion drilling assembly 10 and drillstring 11. Although the bit retainer ring 34 and the retainer sleeve 70 restrict the drill bit 60 from disengaging the driver sub 40, they typically do not restrict the unthreading and disengagement of the driver sub 40 from the case 30.
Once the driver sub 40 and the drill bit 60 are decoupled from the remainder of the percussion drilling assembly 10, the entire drill string 11 must be pulled to replace the dropped bit 60. Further, a fishing operation may be required to retrieve the dropped bit 60. Such tripping and fishing operations undesirably increase the time and cost required to complete the borehole.
Accordingly, there is a need for devices and methods that reduced the likelihood of inadvertent unthreading of the driver sub and case of a percussion drilling assembly. Such devices and methods would be particularly well received if they were relatively inexpensive, simple to manufacture, and did not otherwise interfere with the operation of the percussion drilling assembly.